1. Field of the Invention
This invention relates to an electrode for use in the electrolysis of aqueous solutions of metal halides, etc., especially an electrode suitable for the electrolysis of alkali metal halide solutions of low concentrations and at low temperatures, such as sea water, and to a process for producing the electrode.
2. Description of the Prior Art
An electrolysis device for electrolyzing a dilute salt solution such as sea water to generate chlorine at the anode has previously been used, for example, for preventing adhesion of organisms to underwater structures or for water treatment in swimming pools, city water facilities, and sewage systems. In such an electrolysis, chlorine is usually generated at the anode by using a diaphragm-free electrolysis device, and hypochlorite ion is formed by reaction of chlorine with hydroxyl ion. The product is employed for sterilization, bleaching, etc, in the uses described above. Since such an electrolysis device must be operated continuously for long periods of time with good efficiency and stability, the anode must have an especially high durability while retaining the desired electrode characteristics.
In the electrolysis of sea water or the like, the electrolysis conditions such as the concentration or the temperature of the electrolyte are not constant unlike the case of electrolysis of an aqueous solution of sodium chloride at a relatively high temperature and concentration to produce chlorine and alkali. Furthermore, the temperature of the sea water sometimes decreases to below about 20.degree. C. depending upon natural conditions, the sodium chloride concentration in the brine is usually as low as about 3% by weight, and moreover, a large amount of impurities are dissolved in the brine. Accordingly, electrodes used in this electrolysis should meet various requirements under these conditions, for example, a sufficiently high efficiency for chlorine generation and a sufficiently high durability.
Heretofore, metallic electrodes made by plating a corrosion-resistant substrate with platinum or an alloy of a platinum-group metal are known as electrodes for use in electrolyzing sea water or the like. However, since these electrodes have a relatively high rate of consumption, the thickness of the coating must be increased and the cost of the electrode becomes very high. Furthermore, such electrodes do not have satisfactory electrochemical properties. In electrolysis, the chlorine evolution potential is high, and is scarcely different from the oxygen evolution potential. Accordingly, these electrodes have the defect that the current efficiency is low, and the electrolysis voltage during operation is high.
Various electrodes composed of a corrosion-resistant substrate such as titanium and an electrode coating consisting mainly of an oxide of a platinum group metal, such as ruthenium, are also known as electrodes for use in electrolyzing an aqueous solution of a metal halide such as sodium chloride (for example, as disclosed in U.S. Pat. No. 3,711,385 corresponding to Japanese Patent Publication No. 3954/73). These conventional electrodes, however, do not have entirely satisfactory characteristics for use at low temperatures and low electrolyte concentrations, for example, in the electrolysis of sea water or the like.